The invention relates to surge arresters and other types of electrical power distribution equipment.
Electrical transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. These voltage variations often are referred to as over-voltage conditions.
If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a current path to electrical ground having a relatively low impedance. The surge arrester otherwise operates in a high impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment.
Upon completion of the over-voltage condition, the surge arrester returns to operation in the high impedance mode. This prevents normal current at the system frequency from following the surge current to ground along the current path through the surge arrester.
Conventional surge arresters typically include an elongated outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and electrical ground, and an array of other electrical components that form a series electrical path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high resistance when exposed to a normal operating voltage, and a much lower resistance when exposed to a larger voltage, such as is associated with over-voltage conditions. In addition to varistors, a surge arrester also may include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the varistors. Some arresters also include electrically conductive spacer elements coaxially aligned with the varistors and gap assemblies.
For proper arrester operation, contact must be maintained between the components of the stack. To accomplish this, it is known to apply an axial load to the elements of the stack. Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the elements, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the elements and the end terminals.
One way to apply this load is to employ springs within the housing to urge the stacked elements into engagement with one another. Another way to apply the load is to wrap the stack of arrester elements with glass fibers so as to axially-compress the elements within the stack.
In one general aspect, the invention features a surge arrester or surge arrester module having a stack of components including at least one active electrical element, such as a varistor. Each component has end faces, at least one of which is mechanically bonded to an end face of another component such that the combined components of the stack define a single, monolithic structure that serves as both an electrically-active element and a mechanical support element of the surge arrester. The surge arrester also includes an insulative housing surrounding the stack of components.
The stack of components is capable of withstanding current pulses having magnitudes of 65 kA and durations of 4/10 microseconds, where 4/10 indicates that a pulse takes 4 microseconds to reach 90% of its peak value and 10 microseconds more to get back down to 50% of its peak value, without significant degradation in operating performance of the stack of components.
Embodiments may include one or more of the following features. For example, the stack of components may be capable of withstanding current pulses having magnitudes of 100 kA and durations of 4/10 microseconds without significant degradation in operating performance of the stack of components.
The stack of components may include a first end component, a second end component, and at least one intermediate component. The first end component may include a first end face mechanically bonded to an end face of an intermediate component, and the second end component may include a first end face mechanically bonded to an end face of an intermediate component. The stack of components may also include a pair of conductive end terminals, with a first terminal being mechanically bonded to a second end face of the first end component and a second terminal being mechanically bonded to a second end face of the second end component.
The stack of components may include two or more varistors, and the varistors may be metal oxide varistors (MOVs). At least a first end face of a first varistor and at least a second end face of a second varistor may be covered with metal coatings. The metal coatings may be coatings of aluminum or brass having thicknesses between 0.002 and 0.010 inches.
The varistors may be formed from ceramic material and mechanical bonding between end faces of two adjacent varistors may be provided by stacking the varistors and heating them together such that the mechanical bond is formed by interaction between the adjacent ceramic end faces. The varistors may be unfired, partially fired, or fully fired before they are stacked and heated together.
Mechanical bonding between end faces of two adjacent varistors may be provided by covering a varistor end face with a bond promoting material. The bond promoting material helps to produce a strong, electrically-conductive bond between the varistors. The bond promoting material may be, for example, a slurry of the ceramic material, an organic adhesive, an inorganic adhesive, a metal-filled glass frit, a solder, or a brazing material.
Mechanical bonding between an end face of a varistor and an adjacent component may be provided by applying a metal layer to the end face and attaching the metal layer to a metal surface of the adjacent component. The metal layer and the metal surface may be attached by soldering or brazing. For example, a solder or brazing material having a melting temperature less than 50xc2x0 C. more than an expected operating temperature of the surge arrester may be used.
The metal layer and the metal surface may be attached by stacking the varistor and the adjacent component with a preform element between the metal layer of the varistor and the metal surface of the adjacent component, applying pressure to the varistor and the adjacent component, heating the varistor, the adjacent component, and the preform element to melt the preform element, cooling the varistor and the adjacent component, and removing the applied pressure. The preform element may be formed from a solder composition.
The metal layer and the metal surface also may be attached by coating at least one of the metal layer and the metal surface with an epoxy, stacking the varistor and the adjacent component with the epoxy between the metal layer and the metal surface, applying pressure to the varistor and the adjacent component, heating the varistor and the adjacent component to cure the epoxy, cooling the varistor and the adjacent component, and removing the applied pressure.
Another way of attaching the metal layer and the metal surface includes coating the metal layer and the metal surface with a silver-filled glass matrix, stacking the varistor and the adjacent component with the silver-filled glass matrix between the metal layer and the metal surface, and heating the components.
The adjacent component may be a second varistor and the metal surface may be a surface of a metal layer applied to an end face of the second varistor. The adjacent component also may be a conductive metal terminal and the metal surface may be an end face of the conductive metal terminal.
The surge arrester may satisfy the IEEE Standard for Metal-Oxide Surge Arresters (IEEE Std. C62.11-1999), including the standards applicable to distribution surge arresters.
In another general aspect, the invention features joining end faces of two ceramic varistors by applying a metal layer to an end face of a first varistor, applying a metal layer to an end face of a second varistor, and attaching the metal layers. The metal layers may be attached using soldering or brazing. For example, a solder or brazing material having a melting temperature less than 50xc2x0 C. more than an expected operating temperature of the varistors may be used.
The metal layers may be attached by stacking the varistors with a preform element between the metal layers, applying pressure to the varistors and the preform element, heating the varistors and the preform element to melt the preform element, cooling the varistors and the preform element, and removing the applied pressure.
Similarly, they may be attached by coating at least one of the metal layers with an epoxy, stacking the varistors with the epoxy between the metal layers, applying pressure to the varistors, heating the varistors to cure the epoxy, cooling the varistors, and removing the applied pressure. In yet another approach, the metal layers may be attached by coating the metal layers with a silver-filled glass matrix, stacking the varistors with the silver-filled glass matrix between the metal layers, and heating the components.
Other features and advantages will be apparent from the following description, including the drawings and the claims.